Surface plasmons, also known as surface plasmon polaritons, are electromagnetic waves that propagate at an interface between a metal and a dielectric, in a direction parallel to the boundary between the metal and dielectric. Because the electromagnetic wave propagates on the boundary of the metal and the dielectric, oscillations of the electromagnetic wave may change with irregularities on the boundary, such as, for example, due to the adsorption of molecules to the metal surface. When the surface plasmon encounters a molecule on the metal/dielectric boundary, the molecule may absorb energy from the plasmon and re-emit it as light which is reflected from the metal film.
Surface plasmons may be used to detect molecular adsorption of samples, such as polymers, proteins, etc., by measuring changes in the angle of light reflected from the metal/dielectric boundary. Typically, resonant excitation of surface plasmons (i.e., SPR) is used, where, under resonant conditions, the metal will absorb light energy at a certain angle of incidence (AOI). At this particular AOI, the intensity of the reflected light is decreased, typically evidenced as a sharp dip in the intensity. The AOI at which resonance occurs is affected by the refractive index of the sample layer disposed on the metal. Accordingly, the AOI corresponding to resonance may be used as a direct measure of the characteristics of the sample.